Catalyst having a core and surface layer and use of same in olefin oligomerization

ABSTRACT

PCT No. PCT/EP96/00395 Sec. 371 Date Oct. 27, 1997 Sec. 102(e) Date Oct. 27, 1997 PCT Filed Jan. 29, 1996 PCT Pub. No. WO96/24567 PCT Pub. Date Aug. 15, 1996A molecular sieve comprising a core having deposited thereon a surface layer, wherein the surface layer has a higher Si:Al ratio than that of the core, provides for lower branching in olefin oligomerization products.

FIELD OF THE INVENTION

This invention relates to the treatment of hydrocarbons, especiallyolefinic hydrocarbons, to effect oligomerization, and to catalysts foruse in such treatment.

BACKGROUND OF THE INVENTION

Olefinic hydrocarbons are employed as starting materials in thehydroformylation, or oxo, process, for the eventual manufacture ofnumerous valuable products, e.g., alcohols, esters and ethers derivedtherefrom, aldehydes, and acids. In many of these end uses, linear orlightly branched hydrocarbon chains have advantages compared with moreheavily branched chains.

In the oxo process itself, moreover, olefins with heavily branchedchains are less reactive than those with linear or lightly branchedstructures and, for a given degree of branching, certain isomers areless reactive than others.

Olefinic feedstocks, especially in the C₄ to C₂₀, and more particularlyin the C₆ to C₁₅ range, are frequently produced by oligomerization oflower molecular weight original starting materials, a process that,because of rearrangements that take place during the reaction, mayproduce an undesirably high proportion of multiply branched olefins,even if the original materials are linear. Also, the locations of thebranches, at sites close to each other on the hydrocarbon chain, or inthe central region of the chain, or both, resulting from theoligomerization further reduce the reactivity of the molecules in theoxo reaction.

There are other areas in which a less highly branched hydrocarbon hasadvantages; these include the alkylation of aromatic hydrocarbons byreaction with olefins in the manufacture of surfactants and polyolefinstabilizers.

There is accordingly a need to provide a method to produce an olefinoligomer having a reduced degree of branching of a hydrocarbon material.

U.S. Pat. No. 5,284,989 (Apelian, et al, assigned to Mobil OilCorporation) describes the use of a medium pore size shape-selectiveacid crystalline zeolite in the catalytic oligomerization of olefinichydrocarbons, and discusses the factors influencing the linearity ordegree of branching of the products. Acid activity at the zeoliteparticle surface is said to favour the production of branched products,and reference is made to de-alumination of zeolite surfaces to reducesurface acidity, or the ratio of surface acidity to intracrystallineacid site activity. Other reduction methods mentioned in an extensiveprior art review in the patent include the use of bulky amines toinactivate acid sites; the invention to which the patent is directed isthe use of a dicarboxylic acid to inactivate the surface acid sites.

In U.S. Pat. No. 5,250,484 (Beck et al., also assigned to Mobil),surface acidity is reduced by contacting the catalyst with anammonia-borane solution and calcining to form an inactive ceramic layeron the surface. In U.S. Pat. No. 4,788,374 (Chu et al., also assigned toMobil), surface acidity is reduced by forming a silica shell on ametallosilicate core by crystallizing silica on the surface of the corein the presence of fluoride.

SUMMARY OF THE INVENTION

The present invention provides a process for the oligomerization of anolefin, which comprises contacting under oligomerization conditions afeed comprising at least one olefin with an olefin oligomerizationcatalyst comprising a particulate molecular sieve, each particle of themolecular sieve comprising a core having deposited thereon a surfacelayer, the core comprising a zeolite containing silicon and at least oneelement selected from aluminium, gallium and iron, and the surface layercomprising a zeolite containing silicon and at least one elementselected from aluminium, gallium and iron, the zeolite of the surfacelayer being of the same crystalline structure as the core and having ahigher silicon:selected element ratio than that of the core.

The invention also provides a particulate molecular sieve, capable ofcatalysing olefin oligomerization, each particle of the molecular sievecomprising a core having deposited thereon a surface layer, the corecomprising a zeolite containing silicon and at least one elementselected from aluminium, gallium and iron, and the surface layercomprising a zeolite containing silicon and at least one elementselected from aluminium, gallium and iron, the zeolite of the surfacelayer being of the same crystalline structure as the core and having ahigher silicon:selected element ratio than that of the core.

The invention further provides the use of a particulate molecular sieve,each particle of the molecular sieve comprising a core having depositedthereon a surface layer, the core comprising a zeolite containingsilicon and at least one element selected from aluminium, gallium andiron, and the surface layer comprising a zeolite containing silicon andat least one element selected from aluminium, gallium and iron, thezeolite of the surface layer being of the same crystalline structure asthe core and having a higher silicon:selected element ratio than that ofthe core, as an olefin oligomerization catalyst to reduce the degree ofbranching of the oligomer product.

The invention still further provides a process for the manufacture of aparticulate molecular sieve, which comprises heating an aqueoussynthesis mixture comprising a source of silicon, a source of an elementselected from aluminium, gallium and iron, a source of monovalentinorganic cations, and, if desired or required, an organic structuredirecting agent, the synthesis mixture having dispersed therein crystalsof a molecular sieve containing silicon and an element selected fromaluminium, gallium, and iron, the molar ratio of silicon to selectedelement in the crystals being lower than the molar ratio of silicon toselected element in their respective sources in the synthesis mixture,to cause crystallization of a molecular sieve layer from the synthesismixture onto the surfaces of the crystals.

DETAILED DESCRIPTION OF THE INVENTION

In each of the above-mentioned aspects of the invention, the selectedelement is advantageously aluminium. The elements selected for the coreand for the outer layer are advantageously, but not necessarily, thesame. For example, a gallium-containing outer layer may surround analuminium-containing core.

Advantageously, the resulting crystalline product is ion exchanged withammonium ions or protons, and calcined to yield the acid form of themolecular sieve. Advantageously, calcination takes place at atemperature of at most 600° C., preferably at most 500° C.

Certain features of the process for the manufacture of the molecularsieve of the present invention are shared with processes known in theart as commonly practised or as described in the literature.

These may be described briefly, as follows:

    ______________________________________                                        Propene                                                                       up to 50%                                                                       Propane                                                                       up to 10%                                                                     C                                                                           .sub.4.sup.+                                                                    up to 95%                                                                     Polyunsaturates                                                               up to 1.5%                                                                  ______________________________________                                    

(As used herein, the term polyunsaturates includes compounds having twoor more unsaturated carbon to carbon bonds, whether double or triple,and also compounds other than acetylene which contain one triple bond,e.g., propyne.)

Higher boiling components, especially C₅ ⁺ hydrocarbons, may be removedfrom such a feedstream source as a desirable product, e.g., naphtha fromfluid bed catalytic cracking, as a result of processing or to avoidfurther handling of by-products. For example, the tar formed from steamcracking of vacuum gas oil may be removed as an undesirable by-productin the primary fractionation of the process gas, while the C₅ ⁺component of the product from steam cracking of ethane may be removedduring the quench and process gas compression stages immediatelyfollowing cracking.

Intermediate boiling components (C₃ ⁺) of a feedstream source may alsobe removed from the dilute olefin stream as a desired co-product or toavoid further handling of by-products. For example, propene may beremoved from the process gas effluent of cracked naphtha for use as achemical feedstock. production of the crystalline framework the organiccompound acts as a template around which the crystalline frameworkgrows, or which causes the crystallization to be directed to form aparticular crystalline framework. Preferred agents for the manufactureof ZSM-22 sieves are mono- and di-aminoalkanes having up to 12 carbonatoms, particularly 4, 6, 8, 10 or 12 carbon atoms, e.g.1,6-diaminohexane (which is preferred), diethylamine, 1-aminobutane or2,2'-diaminodiethylamine; arylamines containing up to 8 carbon atoms,heterocyclic organic compounds, e.g., as N-ethylpyridinium;polyalkylenepolyamines, e.g. triethylene tetramine or tetraethylenepentamine, and alkanolamines, e.g. ethanolamine or diethanolamine.

A preferred quantity of template R, based on the preferred template of1,6-diaminohexane, is a molar ratio of R/SiO₂ in the synthesis mixtureof 0.025 to 0.4.

The SiO₂ /Al₂ O₃ molar ratio in the synthesis mixture is generally atleast 150:1, preferably at least 250:1, and may be as high as 1500:1.Ratios between 300:1 and 900:1, especially between 300:1 and 600:1, areespecially preferred.

The SiO₂ /Al₂ O₃ molar ratio in the zeolite layer after crystallizationmay be up to 30% lower than the molar ratio in the synthesis mixture;this reduction may be taken into account in selecting the proportions ofcomponents in the synthesis mixture, to ensure the required relationshipbetween core and outer layer ratios. The SiO₂ /Al₂ O₃ molar ratio in thecore crystals dispersed in the synthesis mixture is advantageously atmost 120:1, is more advantageously in the range 40:1 to 120:1, andpreferably in the range 60 to 100:1.

The proportions of reactants in the synthesis mixture are generallylower than in the normal synthesis mixture, i.e., in addition to thelower aluminium content, the synthesis mixture should be highly diluted,for example, with water. The synthesis mixture including the corematerial may contain, for example, up to 85%, advantageously from 50 to80% by weight, of diluent, especially water.

Advantageously, crystallization is effected at 120 to 180° C.,preferably 140 to 170° C. The crystallization time may be from 10 to 72hours, typically 15 to 48 hours.

After crystallization the zeolite may be washed with deionized water orwith acidified water, and then, optionally after a drying or calciningstep, ion exchanged to yield the acidic form.

The zeolite is preferably exchanged with ammonium ions and subjected toconditions under which the ammonium ions decompose, with the formationof ammonia and a proton, thus producing the acidic form of the zeolite.Alternatively the acid form may be obtained by acid exchange with, forexample, hydrochloric acid.

The exchange with ammonium ions may be carried out by any suitablemethod, for example, by treating the crystals with an aqueous solutionof ammonium chloride, ammonium nitrate or ammonium hydroxide. Exchangewith protons is advantageously carried out by contacting the crystalswith a dilute acid solution, e.g., HCl.

After exchange with ammonium ions or protons, the crystals may becalcined, advantageously at a temperature of from 120° to 600° C.,preferably from 150° to 500° C. Suitable calcination times range from 1hour to several days, the temperatures in the upper part of thespecified temperature range corresponding to the shorter heating timesand the temperatures in the lower part of the specified temperaturerange corresponding to the longer heating times.

Thus, for example, crystals may be calcined at a temperature of 400° C.for from 1 to 20 hours. At a temperature of 120° C., longer calcinationtimes of at least 2 days and preferably from 3 to 5 days will generallybe necessary to achieve adequate voiding of the pores.

The sieve may be post-treated, as by steaming, or may be caused tocontain other cations either by incorporation during preparation or bysubsequent ion-exchange, examples of suitable cations being Ni, Cd, Cu,Zn, Pd, Ca, B and Ti and rare earth metals.

Advantageously, the molecular sieve of the invention has a refinedconstraint index (as hereinafter defined) greater than 2, andadvantageously greater than 10.

The refined constraint index, CI°, is defined in J. A. Martens, M.Tielen, P. A. Jacobs and J. Weitkamp, Zeolites, 1984, p. 98, and P. A.Jacobs & J. A. Martens, Pure and Applied Chem., 1986, Vol. 58, p. 1329,as the ratio of 2-methylnonane to 5-methylnonane produced at 5%conversion in the hydro-isomerization of n-decane.

Examples of molecular sieves having a CI° between 2 and 10 includeZSM-5, 11, 12, 35, 38, 48, and 57, SAPO-11, MCM-22 and erionite, thosehaving a CI° between 5 and 10 presently being preferred. Examples ofmolecular sieves having a CI° greater than 10, and accordingly mostpreferred, include ZSM-22, ZSM-23, and certain ferrierites.

It is within the scope of the oligomerization process of the inventionto employ mixtures containing two or more molecular sieves.

The molecular sieve or zeolite catalyst is advantageously ZSM-22,described in U.S. Pat. No. 4,556,477 and in WO 93/25475, the disclosuresof which are incorporated herein by reference.

A molecular sieve crystallite size advantageously up to 5 μm, preferablywithin the range of from 0.05 to 5 μm, more especially from 0.05 to 2μm, and most preferably from 0.1 to 1.0 μm, may be employed.

The proportion by weight represented by the surface layer, based on thetotal weight of the molecular sieve of the invention, may, for example,be within the range of 5% to 20%, conveniently from 8% to 15%, aftercalcination.

The molecular sieve may be used in the form of granules, powder or othershaped form, e.g., an extrudate. The extrudate advantageously containsthe molecular sieve, and a binder, for example alumina, silica, analuminosilicate, or clay, advantageously in a proportion of from 10:90to 90:10, preferably 20:80 to 80:20, by weight of sieve to binder. Thesieve and binder may be composited by, for example, intimately mixingthem together in the presence of water, and extruding or otherwiseshaping, e.g., by pelletizing.

The feed olefin advantageously contains from 2 to 12 carbon atoms, andpreferably from 2 to 6 carbon atoms; more preferably, the olefin feedadvantageously contains propene, butenes and/or pentenes.

Reaction conditions for the oligomerization process of the invention maybe, with the exception of the use of the novel catalyst, in accordancewith conditions operative for prior art processes oligomerizing the sameolefin.

The olefin may, for example, be fed to the catalyst in admixture with aninert diluent, e.g., a saturated hydrocarbon, in the liquid or,preferably, the gaseous, phase. For a feed comprising propene, asuitable diluent is propane, advantageously in proportions ofpropene:propane from 90:10 to 10:90, preferably from 10:90 to 60:40,especially about 50:50 by weight. Correspondingly, for a butene feed, asuitable diluent is butane, advantageously in proportions from 90:10 to10:90, preferably from 75:25 to 50:50, especially about 2:1, by weightolefin:saturate. The feed is advantageously hydrated; preferably itcontains from 0.05% to 2% by weight water. The desired proportion ofwater may be incorporated by saturating the feed at an appropriatetemperature, e.g., from 25 to 60° C., or by injecting water through apump.

The oligomerization may take place at a temperature advantageously inthe range of from 160° C. to 300° C., preferably from 170° C. to 260°C., and most preferably from 180° C. to 260° C., at a pressureadvantageously in the range of from 5 to 10 MPa, preferably from 6 to 8MPa, and at an olefin hourly space velocity advantageously in the range0.1 to 20, preferably from 0.5 to 10, and most preferably 0.75 to 3.5,whsv.

In olefin oligomerizations employing a normal prior art catalyst, e.g.,ZSM-22, it was found that with a decrease in conversion rate,selectivity to dimer, e.g., from butene to octene, increased but thedegree of branching increased also. Using the catalyst of the presentinvention, however, it has surprisingly been found that, at lowerconversion rates, the selectivity to dimer is retained and isaccompanied by a decrease in the degree of branching. Accordingly,oligomerization may be carried out at a lower conversion rate, unreactedmonomer separated from oligomer, and recycled, resulting in a high dimerselectivity without loss of linearity in the product.

Further, the catalyst of the present invention has the additionaladvantage over the bulky-amine treated material of the prior art that itmay readily be regenerated, as by calcining, without requiring asubsequent amine treatment. The catalyst of the present invention willmoreover not differ on regeneration in its ability to oligomerizeolefins to a less highly branched product from the catalyst of theinvention as originally prepared.

The following examples, in which parts and percentages are by weightunless otherwise stated, illustrate the invention:

EXAMPLES 1 to 3 Preparation of Catalyst Example 1

Preparation of Synthesis Mixture

Solution A

    ______________________________________                                        COMPONENT        PARTS                                                        ______________________________________                                        H.sub.2 O        229.64                                                         Al.sub.2 (SO.sub.4).sub.3 1.8H.sub.2 O 0.6538                                 NaOH (98.4%) 2.11                                                             1,6-diaminohexane 12.85                                                     ______________________________________                                    

The ingredients were dissolved in the water in the order shown.

Solution B

    ______________________________________                                        COMPONENT       PARTS                                                         ______________________________________                                        Ludox AS-40     54.81                                                           (Colloidal Silica)                                                          ______________________________________                                    

Solutions A and B were mixed for about 3 minutes, producing a smoothwhitish gel (synthesis mixture).

Mixture C

    ______________________________________                                        COMPONENT           PARTS                                                     ______________________________________                                        ZSM-22 (H.sub.2 O content 1.18%),                                                                 50.00                                                       SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio 73:1                                  H.sub.2 O 50.09                                                             ______________________________________                                    

Particle size of ZSM-22 ≦1 μm

The components of Mixture C were mixed for 5 minutes, producing a veryviscous paste. 72.54 parts of the synthesis mixture (Solutions A & B)were added and mixed for 15 minutes. An easily pourable and homogenousmass was obtained.

The molar composition of the final synthesis mixture (excluding thecrystals) was:

    26.4 Na.sub.2 O/112.6 R/Al.sub.2 O.sub.3 /372 SiO.sub.2 /26580 H.sub.2 O

where R is 1,6-diaminohexane.

The mixture contained 28.7% preformed ZSM-22 crystals.

80.87 parts of the synthesis mixture were transferred to a stainlesssteel autoclave and heated to 160° C. over a period of 2 hours, and keptat this temperature for 48 hours.

The product was filtered and washed three times with 500 parts water topH 9.4 and subsequently dried at 125° C.; 25.88 parts of dried productwere recovered. The product after drying had an intense yellowishappearance, indicating that the core crystals were covered with asilica-rich ZSM-22 outer layer or shell. The weight ratio of the shellto core was calculated as follows: Parts of synthesis mixture×Fractionpreformed crystals

    ______________________________________                                        (80.87 × 0.287)                                                                           23.21                                                         Parts of Dried Product 25.88                                                  Gain  2.67                                                                    Ratio shell/uncalcined core 2.67/23.21,                                        i.e., 0.12                                                                 ______________________________________                                    

On the assumption that on calcination there is a shell weight loss ofabout 12%, the expected weight ratio of calcined shell to core is about0.10:1.

X-ray diffraction (XRD) on the dried product showed a structure ofZSM-22 very slightly contaminated with crystobalite.

Example 2

Following the procedure of Example 1 a final synthesis mixture of molarcomposition

    53.0 Na.sub.2 O/226 R/Al.sub.2 O.sub.3 /746 SiO.sub.2 /61045 H.sub.2 O

where R is 1,6-diaminohexane, was obtained, containing 29.32% preformedZSM-22 crystals.

111.24 parts of the crystallite-containing synthesis mixture weretransferred to a stainless steel autoclave, which was placed in an ovenat room temperature. The oven was heated to 158° C. over a period of 2hours and maintained at that temperature for 24 hours.

The resulting crystalline product was repeatedly washed with water anddried at 125° C. for 40 hours. 35.8 parts of dry product were recovered.By a calculation as described in Example 1, the weight ratio ofuncalcined shell:core was found to be 0.10:1 and the expected calcinedshell:core ratio was about 0.09:1. XRD showed a pure crystalline ZSM-22structure.

Example 3

Preparation of Synthesis Mixture

Solution A

    ______________________________________                                        COMPONENT        PARTS                                                        ______________________________________                                        Al.sub.2 (SO.sub.4).sub.3 18H.sub.2 O                                                          0.2193                                                         NaOH (98.4%) 2.10                                                             1,6-diaminohexane 12.87                                                       H.sub.2 O 175.00                                                            ______________________________________                                    

The first three components were dissolved in the order shown in the 175parts of water. 54.82 parts of colloidal silica (Ludox AS40) Solution Bwere placed in a mixer, solution A was poured over the mixer contents,and the vessel in which solution A was prepared was rinsed with 54.67parts water, the rinse water then being poured into the mixer. Thecontents were then stirred for 3 minutes to provide Mixture C. To 40.46parts of water were added 44.13 parts of Mixture C, the diluted materialthen being mixed with 35.02 parts of ZSM-22 crystals. After mixing for 5minutes a viscous but pourable mass D resulted, with a molar compositionof:

    78.9 Na.sub.2 O/336 R/Al.sub.2 O.sub.3 /1112 SiO.sub.2 /85320 H.sub.2 O

R being 1,6-diaminohexane; with 29.0% dry weight content of ZSM-22seeds.

110.05 parts of the mass D were transferred to a stainless steelautoclave, which was placed in an oven at room temperature. The oven wasthen heated to 150° C. over 3 hours and maintained at that temperaturefor 24 hours. After the separated crystalline product was washed threetimes to reach a pH (last wash water) of 9.4, it was dried overnight at120° C., yielding 35.75 parts of dried product. Calculation as describedin Example 1 showed a weight ratio of uncalcined shell:core of 0.12:1,and a predicted calcined shell:core of 0.10:1.

In each of Examples 1 to 3, the product was cation exchanged with a 0.5NNH₄ Cl solution, washed, and calcined at 400° C. for 16 hours.

EXAMPLES 4 and 5 AND COMPARATIVE EXAMPLES A, B AND C

Olefin Oligomerization

The following examples were carried out to illustrate the effectivenessof catalysts produced according to the invention in oligomerization ofan olefin. In each case, the feed was a mixed butene feed, diluted withbutanes, in proportions of approximately 65% olefins and 35% saturates,saturated with water vapour at 40° C. Reactor temperature was maintainedin the region of 205 to 235° C., increasing in each case with the numberof days on stream. The reactor pressure was maintained at about 7 MPa.

Prior art catalysts used were (a) the ZSM-22 catalyst employed as corecrystals in Examples 1 to 3 above (termed "Parent" in Tables 1 and 2below), and (b) collidine treated ZSM-22; both catalysts (a) and (b)were formed into extrudates of 5 mm diameter; the catalysts according tothe invention and catalyst (a) were used as powders. Table 1 shows thecatalytic activity in terms of butene conversion at 205° C., 7 MPa andweight hourly space velocity of 1.3 g olefin/g catalyst/hour.

                  TABLE 1                                                         ______________________________________                                        Ex. No.     Catalyst      Conversion, %                                       ______________________________________                                        Comp. A     (a) Parent, powder                                                                          97.0                                                  Comp. B (a) Parent, extrudate 80.3                                            Comp. C (b) Collidine-Treated,  8.1                                            extrudate                                                                    4 Example 1, powder 91.3                                                      5 Example 2, powder 84.9                                                    ______________________________________                                    

EXAMPLES 6 to 13 AND COMPARATIVE EXAMPLES D TO H

In these Examples, the catalysts of Examples 1 and 2 and of ComparisonExamples B and C were used in butene dimerization and the degree ofbranching of the resulting octenes was compared. The feed and conditionsused were as described above with reference to Examples 4 and 5 above,but feed rates and hence space velocities were varied to give differentconversion rates. The results are summarized in Table 2 below.

                                      TABLE 2                                     __________________________________________________________________________              Butene                                                                              Octene Isomers (%) Average                                    Example                                                                            Catalyst                                                                           Conversion                                                                              Mono Di   Tri  Branching                                                                          Selectivity                             No. Example (%) Linear Branched Branched Branched Degree to dimer           __________________________________________________________________________                                            (%)                                   Comp. D                                                                            Comp. B                                                                            74.8  4.2 40.6 50.2 5.0  1.56 49.5                                    Comp. E Comp. B 84.6 3.9 39.3 52.3 4.5 1.57 48.4                              Comp. F Comp. B 90.2 4.3 42.2 49.8 3.7 1.53 45.0                              Comp. G Comp. B 96.1 5.8 47.4 44.9 2.0 1.43 31.7                              Comp. H Comp. C 73.5 8.4 66.4 23.4 0.8 1.18 58.2                               7 1 73.9 6.1 56.3 35.1 2.5 1.34 58.2                                          8 1 79.5 5.6 54.6 37.2 2.5 1.37 57.8                                          9 1 91.3 4.2 45.2 47.0 3.5 1.50 48.5                                         10 1 93.4 4.7 47.1 46.0 2.2 1.46 46.1                                         11 2 72.7 5.5 58.1 33.1 3.3 1.34 56.9                                         12 2 81.1 5.5 56.0 35.9 2.7 1.36 61.1                                         13 2 91.5 5.4 52.2 40.5 1.9 1.39 56.5                                         14 2 95.0 6.8 63.4 27.8 1.9 1.25 47.3                                       __________________________________________________________________________

We claim:
 1. A particulate molecular sieve, each particle of themolecular sieve comprising a core having deposited thereon a surfacelayer, the core comprising a zeolite containing silicon and aluminium,and the surface layer comprising a zeolite containing silicon andaluminium, the zeolite of the surface layer being of the samecrystalline structure as the core and having a higher silicon:aluminiumratio than that of the core.
 2. The particulate molecular sieveaccording to claim 1, wherein the Si:Al ratio in the surface layer is inthe range 300:1 to 1500:1.
 3. The particulate molecular sieve accordingto claim 1, wherein the molecular sieve has a CI° of at least
 10. 4. Aprocess for the oligomerization of an olefin, which comprises contactingunder oligomerization conditions a feed comprising at least one olefinwith an olefin oligomerization catalyst comprising a particulatemolecular sieve comprising a core having deposited thereon a surfacelayer, the core comprising a zeolite containing silicon and at least oneelement selected from aluminium, gallium and iron, and the surface layercomprising a zeolite containing silicon and at least one elementselected from aluminium, gallium and iron, the zeolite of the surfacelayer being of the same crystalline structure as the core and having ahigher silicon:selected element ratio than that of the core.
 5. Theprocess according to claim 4, wherein the silicon:selected element ratioof the core is at most 120:1.
 6. The process according to claim 4,wherein the silicon:selected element ratio of the core is in range 60:1to 100:1.
 7. The process according to claim 4, wherein thesilicon:selected element ratio in the surface layer is at least 150:1.8. The process according to claim 4, wherein the silicon:selectedelement ratio in the surface layer is in the range 300:1 to 1500:1. 9.The process according to claim 4, wherein the molecular sieve has a CI°of at least
 2. 10. The process according to claim 4, wherein themolecular sieve has a CI° of at least
 10. 11. The process according toclaim 4, wherein the molecular sieve is ZSM-22.
 12. The processaccording to claim 4, wherein the selected element is aluminium.
 13. Theprocess according to claim 4, wherein the olefin contains from 2 to 12carbon atoms.
 14. The process according to claim 4, wherein the olefincontains from 2 to 6 carbon atoms.
 15. The process according to claim 4,wherein said process is carried out at a temperature within the range offrom 160° C. to 300° C.
 16. The process according to claim 4, whereby anoligomer having a reduced degree of branching is formed.
 17. A processfor the manufacture of a particulate molecular sieve, which comprisesheating an aqueous synthesis mixture comprising a source of silicon, asource of aluminium, a source of monovalent inorganic cations, thesynthesis mixture having dispersed therein crystals of a molecular sievecontaining silicon and aluminium, the molar ratio of silicon toaluminium in the crystals being lower than the molar ratio of silicon toaluminium in their respective sources in the synthesis mixture, to causecrystallization of a molecular sieve layer from the synthesis mixtureonto the surfaces of the crystals.
 18. The process according to claim17, wherein said aqueous synthesis mixture further comprises an organicstructure directing agent.